Thyroid hormones mediate responses through receptors (Trs) that are members of the nuclear receptor superfamily, including receptors that bind steroids, vitamins and fatty acids, each a major pharmaceutical target. We utilized X-ray crystallographic information about the TR ligand-binding domain (LBD) to guide placements of mutations on the TR LBD surface that block binding of proteins that interact with it. These are preferable to randomly placed mutations that often disrupt receptor folding and complicate interpretations. Using this scanning surface mutagenesis approach we defined the coactivator-binding surface, a distinct cleft which now provides a novel pharmaceutical target and provides information about ligand-induced conformational changes mediating receptor action. We defined dimer and heterodimer binding surfaces, which provide insights into formation and functions of TR homodimers vs. heterodimers with retinoid-X receptors (RXRs), and how receptors bind DNA. We defined a corepressor-binding surface. In the proposed studies we plan to use our large and growing bank of mutations to extend analyses to additional proteins and examine interactions of Trs with a selected group of proteins and their functional consequences in greater detail. Native and mutated Trs will be examined for protein binding in vitro and regulatory activities in cell culture. Proteins to be examined include: a novel coactivator, ligand-dissociated corepressors, a ligand-dependent inhibitor, and a receptor that blocks TR action. We will also use mutant receptors to: (i) define roles of the dimer surface and the hinge region connecting the LBD with the DNA-binding domain in formation of dimers and heterodimers with RXRs, mediation of allosteric influences, and influences on TR-DNA binding; and (ii) define TR surfaces involved in ligand-dependent repression and ligand-independent activation. Overall, these studies endeavor to provide a comprehensive understanding of the nuclear receptor LBD surface, with insights into receptor transmission of unliganded and hormone-mediated regulation of gene expression. The information gained will be relevant for understanding the nuclear receptor superfamily and gene regulation mechanisms in general.